In order to improve paper properties and reduce manufacturing costs in papermaking, various additives are applied to the pulp slurry prior to sheet formation or after an initial drying of the paper. Those additives applied to the pulp in an aqueous slurry are known as wet-end additives and include retention aids to retain fines and fillers, for example, alum, polyethylene imine, cationic starches and the like; drainage aids, such as polyethylene imine; defoamers; and pitch or stickies additives, such as microfibers and adsorbent fillers. Other wet-end additives include polymers such as, cationic polyarylamides and poly(amide amine/epichlorohydrin) which are added to improve wet strength as well as dry strength of the paper. Starch, guar gums, and polyacrylamides are also added to yield dry strength improvements. Sizing agents are occasionally added to impart hydrophobic character to the hydrophilic cellulosic fibers. These agents are used in the manufacture of paper for liquid containers, for example, milk or juice, paper cups and surfaces printed by aqueous inks where it is desired to prevent the ink from spreading. Such sizing agents include rosin sizes derived from pine trees, wax emulsions and, more recently, cellulose-reactive sizes.
The application of additives to paper after an initial drying of the sheet by spraying, capillary sorption, immersion, roll-coating, and the like, is often referred to as a dry-end addition. Poly(vinyl alcohol), acrylic or vinyl acetate emulsions, starches, sizing agents, polyurethanes, and SBR latex are commonly added at the dry end.
Improvements are continually sought in wet-strength additives for paper. Improved speed of wet-strength development is desired and many wet strength additives require both time and temperature to develop their wet strength properties. Initial wet strength is desired to improve the wet web strength during paper formation. A review of the utility of paper additives is given by G. G. Spence, Encyclopedia of Polymer Science and Technology, Second Edition, Wiley-Interscience, Vol. 10, pgs. 761-786, New York (1987).
The use of functional polymers of various types has been known for many years as a means to improve papermaking processes and paper properties. Several of these resins for improving wet strength of the paper have involved products derived from epihalohydrin. U.S. Pat. No. 3,535,288 Lipowski, et al. (1970) discloses an improved cationic polyamide-epichlorohydrin thermosetting resin as useful in the manufacture of wetstrength paper. U.S. Pat. No. 3,715,336 Nowak, et al. (1973) describes vinyl alcohol/vinylamine copolymers as useful flocculants in clarification of aqueous suspensions and, when combined with epichlorohydrin, as useful wet-strength resins for paper. The copolymers are prepared by hydrolysis of vinylcarbamate/vinyl acetate copolymers made by copolymerization of vinylacetate and vinyl isocyanate followed by the conversion of the isocyanate functionality to carbamate functionality with an alkanol. Additionally, Canadian Patent No. 1,155,597 (1983) discloses wet-strength resins used in papermaking, including polymers of diallylamine reacted with epihalohydrin and a vinyl polymer reacted with epihalohydrin wherein the vinyl polymer is formed from a monomer prepared by reacting an aromatic vinyl alkyl halide with an amine, such as dimethylamine.
Functional polymers derived from amides have also been used to improve paper processes. U.S. Pat. No. 3,597,.314 Lanbe, et al. (1971) discloses that drainage of cellulose fiber suspensions can be enhanced by the addition of a fully or partially hydrolyzed polymer of N-vinyl-N-methyl carboxylic acid amide. U.S. Pat. No. 4,311,805 Moritani, et al. (1982) discloses paper-strength additives made by copolymerizing a vinyl ester, such as vinyl acetate, and an acrylamide derivative, followed by hydrolysis of the ester groups to hydroxy groups. The presence of the remaining cationic groups enables the polymer to be adsorbed on pulp fibers. Utilities for the polymers as sizing agents, drainage aids, size retention aids and as binders for pigments are disclosed but not demonstrated. U.S. Pat. No. 4,421,602 Brunnmueller, et al. (1983) describes partially hydrolyzed homopolymers of N-vinylformamide as useful as retention agents, drainage aids and flocculants in papermaking. European Patent Application 0,331,047 (1989) notes the utility of high molecular weight poly(vinylamine) as a wet-end additive in papermaking for improved dry strength and as a filler retention aid.
More recently, vinylamide copolymers have been disclosed as useful in papermaking to improve the properties of the product. U.S. Pat. No. 4,774,285 Pfohl, et al. (1988) describes amine functional polymers formed by copolymerizing vinyl acetate or vinyl propionate with N-vinylformamide (NVF) followed by 30-100% hydrolysis to eliminate formyl groups and the acetyl or propionyl groups. The copolymer contains 10-95 mole % NVF and 5-90 mole % vinyl acetate or vinyl propionate. The hydrolyzed copolymers are useful in papermaking to increase dry strength and wet strength when added in an amount of 0.1 to 5 wt % based on dry fiber. The polymer can be added to the pulp or applied to the formed sheet. The two polymers used to show dry and wet strength improvements are said to contain 40% and 60% N-vinylformamide before hydrolysis. Lower levels of amine functionality in poly(vinyl alcohol) are not demonstrated to be effective.
U.S. Pat. No. 4,808,683 Itagaki, et al. (1989) describes a vinylamine copolymer such as a copolymer of N-vinylformamide and N-substituted-acrylamide, which is said to be useful as a paper strengthening agent and European Patent Application 0,251,182 (1988) describes a vinylamine copolymer formed by hydrolysis of a copolymer of N-vinylformamide and acrylonitrile or methacrylonitrile. The product is said to be useful in papermaking as a drainage aid, retention aid and strength increasing agent. Examples presented to demonstrate the paper strengthening effect of the polymer used a pulp slurry containing cationic starch, alkyl ketene dimer as a sizing agent and a filler retention improving agent, but there is no indication of any cooperative effect between the polymer and the sizing agent.
On the other hand, certain combinations of additives have been found to be useful as paper additives. U.S. Pat. No. 4,772,359, Linhart, et al. (1988) discloses utility of homopolymers or copolymers of N-vinylamides, such as N-vinylformamide (NVF), in combination with phenol resin as a drainage aid in pulp slurries for production of paper. In this service unhydrolyzed poly NVF is said to function cooperatively with the phenol resin, while a partially hydrolyzed poly NVF does not (see Example 6). European Patent Application No. 0,337,310 (1989) describes improving moist compressive strength of paper products using the combination of hydrolyzed poly(vinylacetate-vinylamide) and an anionic polymer such as carboxymethyl cellulose or anionic starch. The hydrolyzed polymer can contain 1-50 mole % vinylamine units and examples are given of polymers having amine functionality of 3-304.
The contribution of Spence to the Encyclopedia of Polymer Science and Engineering, noted above, provides a comprehensive survey of paper additives describing the functions and benefits of various additives and resins used in the manufacture of paper. Wet-end additives are discussed at length. Resins containing amine groups that provide cationic functionality and have low molecular weights (10.sup.3 to 10.sup.5) e.g., polyethylene imine, are used to aid retention of fines in the paper. Acrylamide-based water soluble polymers are used as additives to enhance dry strength of paper while a variety of resins, such a melamine-formaldehyde resins, improve wet strength. Polyethylene imine, however, is said not to be commercially significant as a wet-strength resin. Sizing agents are used to reduce penetration of liquids, especially water, into paper which, being cellulosic, is very hydrophilic. Sizing agents disclosed are rosin-based agents, synthetic cellulose-reactive materials such as alkyl ketene dimer (AKD), alkenyl succinic anhydrides (ASA) and anhydrides of long-chain fatty acids, such as stearic anhydride, wax emulsions and fluorochemical sizes. Cationic retention aids, such as alum, cationic starch or aminopolyamide-epichlorohydrin wet-strength resin, are used to retain the size particles in the sheet.
Martono TAPPI J., pages 139-43 (November 1990) discusses alkyl ketene dimer reactions and points out that hydrolysis is a competing reaction to the esterification reaction between AKD and cellulose, reducing the effectiveness of the size. The AKD size emulsions were stabilized with cationic starch or polyamine amide-epichlorohydrin resin, the latter exhibiting much higher hydrolysis rates. Both AKD and ASA form covalent ester bonds with cellulose-OH groups, but react also, depending upon conditions, with other OH groups in the surrounding medium, foremost through hydrolysis with water.
Zhou, Paper Technology, pages 19-22 (July 1991) discusses AKD sizing studies which suggest that AKD sizing increases over a period of time after application, particularly at elevated temperatures.
U.S. Pat. No. 5,114,538 Malatesta (1992) discloses a process for sizing paper and similar products using cyclic acid anhydrides as such or in solution with non-aqueous protic and/or protic solvents. U.S. Pat. No. 2,916,366 Weisgerber (1960) discloses a method of sizing paper by adding an aqueous emulsion of ketene dimer to a dilute aqueous pulp suspension along with from about 0.0014 to about 0.2% poly(vinylamine). U.S. Pat. No. 4,784,727 Schroer, et al. (1988) discloses sizing agents containing from 1 to 60 parts, by weight, of a fixing and sizing accelerating agent and from 0 to 80 parts, by weight, of conventional auxiliaries per 10 parts by weight of hydropholic, cellulose-reactive sizing materials.